Fuel cells utilize the chemical conversion of a fuel with oxygen to water, in order to generate electric energy. For this purpose, fuel cells contain the so-called membrane electrode unit (MEA for membrane electrode assembly) as the core component, which is an arrangement of an ion-conducting (mostly proton-conducting) membrane and a catalytic electrode (anode and cathode) situated on each side of the membrane, respectively. The latter include mostly supported precious metals, in particular, platinum. In addition, gas diffusion layers (GDL) may be situated on each side of the membrane electrode unit on the side of the electrodes facing away from the membrane. The fuel cell is generally formed by a plurality of MEAs assembled in a stack, the electric powers of which are cumulative. Bipolar plates (also called flow field plates), which ensure that the individual cells are supplied with operating media, i.e., reactants, and are normally also used for cooling, are generally situated between the individual membrane electrode units. The bipolar plates also ensure an electrically conductive contact to the membrane electrode units.
During operation of the fuel cell, the fuel, in particular, hydrogen H2 or a hydrogen-containing gas mixture, is fed to the anode via an open flow field of the bipolar plate on the anode side, where an electrochemical oxidation of H2 to H+ and simultaneous discharge of electrons takes place. A (water-bound or water-free) transport of the protons H+ takes place from the anode chamber into the cathode chamber via the electrolytes or the membrane, which separates the reaction chambers from one another in a gas-tight manner and electrically isolates them. The electrons provided at the anode are conveyed to the cathode via an electric line. The cathode is supplied with oxygen or an oxygen-containing gas mixture (for example, air) via an open flow field of the bipolar plate on the cathode side, so that a reduction of O2 to 2 O2− under absorption of the electrons takes place. At the same time, the oxygen anions react in the cathode chamber with the protons transported via the membrane while forming water.
During operation of fuel cell stacks in the low load range, as occurs, for example, in fuel cell vehicles, in particular, in urban traffic, load points are frequently driven, which correspond to a single cell voltage above 0.8 volts. Such high voltages result in an oxidation of the catalytic material, in particular of the cathode electrode, in which platinum reacts to form platinum oxide, which is significantly less reactive for the catalytic oxygen reduction than metallic platinum. In addition, the aforementioned voltages cause the platinum to dissolve into very small quantities, which enter into cationic solution. Thus, high single cell voltages of the fuel cell result on the whole in a loss of catalytic activity and available catalyst surface and, therefore, in a loss of fuel cell efficiency.
To counteract this phenomenon, the attempt is made in modern fuel cell vehicles to avoid the presence of stack voltages which correspond to a single cell voltage greater than 0.85 volts by way of a permanent, minimal load requirement of a few kW. It has been found, however, that during actual operation, the voltages obtained in the entire operating range nevertheless frequently lie above the specified voltage values and, therefore, in the harmful range.
From JP 2013-243047 A, it is known to avoid high output voltages of fuel cells which may cause damage to electrode catalysts. If a requested voltage corresponding to a requested power exceeds an upper cut-off voltage and also increases over time, the output voltage below the upper cut-off voltage is limited and a battery is charged with the surplus current.
According to JP 2008-130424 A, it is checked in the case of an increasing power requirement whether this could cause damage to the catalyst. If this is the case, the fuel cell is controlled in such a way that the output power is raised with a gradient slower than required and the missing power is supplemented by the battery.
WO 2008/111654 A1 (=DE 11 2008 597 B4) describes a method for activating the catalyst of fuel cells, in which the output voltage of the fuel cell is lowered to a level at which the oxides of the electrode catalyst are reduced. The method is carried out in a stationary operating situation, in which the required amount of power of the entire system is small and none of the fuel cell power is directly supplied to a traction motor, and the gas pedal of the vehicle is not actuated. The surplus power generated by the voltage drop is used preferably for charging the battery or is supplied to electrical auxiliary consumers of the vehicle.